The UK will see the mercury “creeping down” as the days go by but temperatures will remain above average through the week, the Met Office said.
Monday will see temperatures peaking at about 14C in Lancashire, with the possibility of scattered showers and “blustery” winds in certain areas.
“The showers should die out through the afternoon, promising a fine end to the day,” the Met Office added.
It comes after the hottest day of the year so far on Friday, with a high of 23.4C (74.12F) recorded at St James’s Park in London.
Saturday saw another warm day, not far shy of the year’s record, with 21.2C (70.16F) recorded in Cardiff.
Easter Sunday brought the start of the decline in temperatures, with a slightly cooler high of 20.2C (68.36F) in Northolt, west London.
Tuesday could even bring some frost for Northern Ireland and the south and east of Scotland followed by a day of sunshine and showers across most of the UK, with temperatures dropping again slightly to 17C (62.6F) or 18C.
The Met Office said Wednesday will be a “similar” picture, with temperatures continuing to slowly decrease into the end of the week.
The dip comes as a cooler front arrives from the west followed by an easterly breeze later in the week.
Met Office senior meteorologist Marco Petagna said “the main thing is with each day since Friday we’ve seen maximum temperatures just creep down”, adding the end of the week will bring a “cooler, cloudier and breezier picture”.